Particle - shader question

Here' what I want to accomplish ...
Run a particle stream of spheres and have the spheres fade out after 2-3 seconds. Not the entire stream of particles - only the ones that have been in existence for the spec ...

Edit: Not sure what happened - message got cut off!!!

Fade out only the spheres that have been in existence for the specified time.
I know how to limit lifetime, fade out the entire emitter using a shader and change size over time.
But can't figure out how to make them fade away.

In my second try I simplified the alpha channel. The particle age is modified by the curve filter so that for the first 70% of the lifespan will have alpha 100%. Then it will fall to 0% by the end of the lifespan.

If you are interested in my test scene, PM me your email and I'll send it to you.

Never used the curve filter before - opens up a whole new bag of tricks!

Was just starting to try Shoestring Shaders proximity shader - a powerful tool with it's own WOW factor, but this gives me the result I was looking for.
Applied the same concept to the glow channel and now I can get my particles to change glow color at midstream.
Will have to keep experimenting.